A wall surface of a boiler furnace is constituted by heat transfer pipes and is provided with a number of burners which burn pulverized coal, petroleum or other fuel in the furnace.
FIG. 1 shows a schematic diagram of a boiler which uses pulverized coal as fuel.
In FIG. 1, reference numeral 1 denotes a coal burning boiler furnace. In a lower portion of the furnace 1, pulverized coal burner groups 2 are arranged on plural stages (three stages are shown in FIG. 1). Each of the groups 2 includes a required number of pulverized coal burners 3 arranged horizontally along the wall surface.
Arranged above (downstream of) the pulverized coal burner groups 2 are over air port groups 4 on required stages (shown as one stage in the figure). Each of the groups 4 is constituted by a required number of over air ports 5 arranged horizontally. The over air ports 5 are arranged vertically above the corresponding pulverized coal burners 3.
The pulverized coal burner groups 2 are supplied with combustion air through combustion air supply passages 6 and 7. Supplied to the over air port groups 4 is two-step-combustion air through an over-air-port air combustion passage 8 branched from the supply passage 6. The pulverized coal burners 3 are supplied with pulverized coal from a coal pulverizer (not shown) along with combustion air.
In the furnace 1, pulverized coal is injected and burned along with one-step-combustion air from the pulverized coal burner groups 2. Further, the two-step-combustion air is injected from the over air port groups 4 and is mixed with a combustion gas to reduce NOx and facilitate combustion of a solid unburned portion (char) in the combustion gas; and further CO gas is burned.
Dampers 9 and 10 for airflow rate adjustment are incorporated in the combustion air supply passage 7 connected to the pulverized coal burners 3 and in the over-air-port air combustion passage 8 connected to the over air ports 5, respectively.
An example of a conventional burner will be described in terms of the pulverized coal burner 3 with reference to FIG. 2.
In FIG. 2, reference numeral 1 denotes a furnace; and 12, a wall of the furnace 1.
The furnace wall 12 has a throat 13. Attached to the furnace wall 12 on a side away from the furnace 1 is a wind box 14 which houses the pulverized coal burner 3 concentrically of the throat 13. The wind box 14 is connected with the combustion air supply passage 7.
The pulverized coal burner 3 comprises a nozzle body 16 and a secondary air adjuster 17 surrounding a leading end (an end near the furnace) of the nozzle body 16.
The nozzle body 16 comprises concentric outer and inner cylinder nozzles 18 and 19 and an oil burner 20 arranged axially of the nozzle 19. The outer and inner cylinder nozzles 18 and 19 have circular cross-sections to define together a fuel conduction space 21 as a hollow cylindrical space with an open end near the furnace 1.
Tangentially communicated with a base (an end away from the furnace 1) of the outer cylinder nozzle 18 is a primary air induction pipe 22 connected to a coal pulverizer (not shown). Through the induction pipe 22, primary air 24 and pulverized coal entrained thereon flow tangentially into and swirl in the fuel conduction space 21 and are injected through a leading end of the space 21.
Opened to a base of the inner cylinder nozzle 19 is an end of a tertiary air induction pipe 23 the other end of which is opened to the wind box 14 so as to take in and guide combustion air delivered to the wind box 14 to the inner cylinder nozzle 19 as combustion auxiliary air, i.e., tertiary combustion air.
The secondary air adjuster 17 comprises an auxiliary air adjustment mechanism 25 which houses a leading end of the nozzle body 16, and a main air adjustment mechanism 26 arranged concentrically outside of the adjustment mechanism 25 in an overlapping manner.
The auxiliary air adjustment mechanism 25 comprises a first air guide duct 28 reduced in diameter toward the leading end and a number of inner air vanes 29 arranged pivotally. The inner air vanes 29 are synchronously pivotable through a link mechanism (not shown) to change their tilt angle to air flow. The main air adjustment mechanism 26 comprises a second air guide duct 32 reduced in diameter toward the leading end and a number of outer air vanes 33 arranged pivotally and circumferentially equidistantly. The outer air vanes 33 are synchronously pivotable through a link mechanism (not shown) to change their tilt angle to the air flow as is the case with the inner air vanes 29.
The leading end of the second air guide duct 32 is contiguous with the throat 13. The leading end of the first air guide duct 28 is set back from an inner wall surface of the furnace wall 12. The leading ends of the cylinder nozzles 18 and 19 are further set back from the leading end of the first air guide duct 28.
Combustion in the above-mentioned pulverized coal burner 3 will be briefly described. Pulverized coal is supplied along with the primary air 24 from the primary air induction pipe 22 to the base of the fuel conduction space 21. The primary air 24 flows toward the furnace 1 while swirling in the space 21, is contracted during its passage through the space 21 and is injected through the leading end of the outer cylinder nozzle 18. Secondary air 34, which is auxiliary combustion air raised to a required temperature, is supplied to the wind box 14. The secondary air 34 is swirled by the outer air vanes 33 and injected through the second air guide duct 32 to the furnace 1 along with the primary air 24 and the pulverized coal.
In the course of injection to the furnace 1, the pulverized coal is uniformized by swirling in the space 21, raised in temperature by the secondary air 34 and further heated by receiving radiation heat from the furnace 1. Such heating causes the pulverized coal to release a volatile content which is ignited to continuously maintain flames.
A portion of the secondary air 34 taken into the second air guide duct 32 is taken into the first air guide duct 28 through the inner air vanes 29 and is injected as secondary auxiliary air. The inner air vanes 29 are tilted to the air flow to swirl the taken portion of the secondary air 34.
A state of a supply flow rate of the secondary air 34 is changed by airflow rate adjustment by the outer air vanes 33 and swirling strength and airflow rate adjustments by the inner air vanes 29 to thereby adjust a combustion state of the pulverized coal.
Moreover, a portion of the secondary air 34 is guided as tertiary air 35 through the tertiary air induction pipe 23 to the inner cylinder nozzle 19 and is injected through the inner cylinder nozzle 19. The combustion state of the pulverized coal is adjusted by injecting the tertiary air 35. Thus, the combustion state of the pulverized coal is optimized by the adjustments of the secondary and tertiary airs 34 and 35, etc.
In the above-mentioned conventional pulverized coal burner 3, the outer and inner air vanes 33 and 29 are coupled by their respective link mechanisms so that higher processing accuracy of parts and delicate assembly adjustment by a skilled mechanic are required for accurate assembling without backlash, which increase manufacturing cost and make cost reduction difficult.
Backlash, which inevitably increases over time in the link mechanisms, brings about variation of the tilt angles of the inner and outer air vanes 29 and 33 from the initial setting, leading to significant variation in swirling strength. Change of the angles of the inner and outer air vanes 29 and 33 for compensation of the airflow rate and the swirling strength is problematic in that an input angle does not correspond to an actual change amount and that a time-lag occurs upon change of an angle of the vanes. Thus, it is considered that highly accurate combustion control may become difficult.
A general technical level of burners is disclosed, for example, in JP 58-127005A.